In july, Wii has "only" sold 4 times more than PS3, when in june it was 6 times, in may 5 times, and in april... 4 times... rather than a "decline", i'd call this a very normal situation, with very steady Wii sales, and a just slightly better situation for the PS3...
It's very simple, just look at the vgchartz Japan's hardware sales graphic:
http://www.vgchartz.com/hwcomps.php?cons1=Wii®1=Japan&cons2=PS3®2=Japan&cons3=X360®3=Japan&align=1
You see that blue line? That's the Wii...
Is it falling? Even just a little bit? If you have good eyes, you have a good answer...
"A beautiful drawing in 480i will stay beautiful forever...
and an ugly drawing in 1080p will stay ugly forever..."